The trade in the kingdom report is from treaties and such. Caravan bonuses don't show up there.
As for the bonus... it can be substantial. I've reached over 100% bonus on a single caravan which, on a city of a lot of base food production, is enough to sustain itself and then some. Note that the bonus applies to both ends so if you have two cities producing food and connect them, you would get the bonus applied to both cities. As well, a city can have more than one caravan send to it.
Give or take (I can check later), one large map square will probably give you in the 5-20 range. Two will be 40-60. At max zoom (at least for me) from one side of the window to the other, it's 100 to 120.
I am almost certain that the bonus is based on road distance not physical distance so your mileage may vary.